1. Field of the Invention
The present invention relates in general to the field of signal processing, and more specifically to a system and method for providing overload protection and stability in high order, 1-bit delta-sigma modulators.
2. Description of the Related Art
Many signal processing systems, such as audio signal processing systems, utilize delta-sigma modulators to provide output data with a high, in-band signal to noise ratio. FIG. 1 depicts a conventional digital signal processing (xe2x80x9cDSPxe2x80x9d) system 100 that includes a P-order digital loop filter 112 with a z-domain transfer function of L(z). DSP system 100 receives an R-bit digital input signal 104 from source 106. Source 106 can be any digital signal source, such as an analog-to-digital converter, a compact disk player and/or recorder system or a digital versatile disk (DVD) player and/or recorder. Digital input signal 104 often undergoes one or more preprocessing operations 108 prior to modulation by delta-sigma modulator 102. The type of pre-processing operation depends upon the purpose of DSP system 100. Example preprocessing operations are decimation and interpolation. Often preprocessing operation(s) 108 increase or decrease the sampling frequency of digital input signal 104 by a factor of xe2x80x9ckxe2x80x9d. A summer 110 adds the now k-bit digital input signal 104 with the negative of output data q from delta-sigma modulator 102. The P-order digital loop filter processes the oversampled digital input signal 104 minus feedback signal q and generates an output signal u.
Delta-sigma modulator 102 represents a one-bit delta-sigma modulator. One-bit delta-sigma modulators provide a full-scale xe2x80x9chighxe2x80x9d or full scale xe2x80x9clowxe2x80x9d quantization output data q. The full-scale xe2x80x9chighxe2x80x9d and full-scale xe2x80x9clowxe2x80x9d are referred to herein as respective logical values +1 and xe2x88x921. Thus, if the input signal, u, to the 1-bit quantizer 114 resides above a predetermined threshold level, q is +1. Output data q is otherwise xe2x88x921. Output data q provides feedback to loop filter 112. Output data q is provided to one or more post-processing components 116. The output data q can be used directly by some digital recording components to directly encode digital media. In other embodiments, post-processing components include a digital-to-analog converter (xe2x80x9cDACxe2x80x9d), such as a switched capacitor filter, to process groups of M output signals q to reconstruct the digital input signal 104 into an analog waveform. Post-processing components 116 can also include an analog low pass filter to filter out out-of-band noise and provide the filtered output signal to a load, such as a speaker system. xe2x80x9cDelta-sigma modulatorsxe2x80x9d are also commonly referred to using other interchangeable terms such as xe2x80x9csigma-delta modulatorsxe2x80x9d, xe2x80x9cdelta-sigma convertersxe2x80x9d, xe2x80x9csigma delta convertersxe2x80x9d, and xe2x80x9cnoise shapersxe2x80x9d.
Many DSP systems utilize delta-sigma modulators because of their noise shaping capabilities. The delta-sigma modulator 102 shapes the noise frequency components of digital input signal 106 so that nearly all the noise energy lies out-of-band, i.e. outside a bandwidth of interest. In an audio system, the bandwidth of interest typically lies within the range of 0-25 kHz. P-order loop filter 112 is well-known to those of ordinary skill in the art and includes P integration stages. In general, higher order delta-sigma modulators provide better in-band noise shaping characteristics by shifting more noise energy to frequencies outside the bandwidth of interest. The inherently coarse quantization of 1-bit delta-sigma modulators results in a higher noise floor. Because of the better in-band noise shaping characteristics of higher order loop filters, 1-bit delta-sigma modulators often include at least a third order loop filter. Third order and higher order delta-sigma modulators are referred to herein as xe2x80x9chigh orderxe2x80x9d delta-sigma modulators.
Stability is a primary concern in the design and operation of delta-sigma modulators. xe2x80x9cInstabilityxe2x80x9d of a delta-sigma modulator means that the delta-sigma modulator exhibits large, bounded (or unbounded) states and poor signal to noise ratios, especially as compared with predictions by linear models. Stability in high order delta-sigma modulators is even more challenging than 1st and 2nd order delta-sigma modulators because the analytical complexities of high order delta-sigma modulators in actual operational environments have defied conventional attempts to develop accurate instability prediction mechanisms. Thus, although empirical evidence can be obtained to suggest stability for high order delta-sigma systems over a variety of operational regions, stability cannot be guaranteed for all possible input signals.
Many causes for instability exist with high order delta-sigma modulators. One such cause is quantizer overload. Quantizer overload generally occurs when a quantizer receives input data that is either excessively high or low. Higher order delta-sigma modulator systems exhibit an increasingly lower tolerance to input signals that approach reference levels (i.e. output limits) of the quantizer. For example, an input signal as small as one-half of a reference level can cause a sixth order delta-sigma modulator to overload. Quantizer overload conditions can cause instability via a number of mechanisms including nonlinear feedback to the high order loop filter, large quantization noise, and low signal gain. For example, if quantizer 114 has a limited range of output signals, such as signals between reference levels of +/xe2x88x925. An input signal outside the range will cause the quantizer 114 to unsuccessfully attempt to track the output signal. Signals can be unintentionally outside the range due to a variety of well-known reasons including the Gibbs phenomenon overshoot that occurs when a discontinuity exists between input samples. Even when the input signal falls within the range of quantizer 102 but lies near the range boundary, the quantizer 102 could overload. The output signal of delta-sigma modulator 102 can generally be approximated by a probability density function having an approximately Gaussian distribution. Thus, although the input signal 104 to the delta-sigma modulator 102 may originally lie within a non-overload range, the input signal u to the 1-bit quantizer 114 may exceed the non-overload range. Accordingly, quantizer overload conditions can exist when a probability of quantizer overload exists. Such error can be attributed to a number of sources such as internal noise and other system perturbations. Thus, if the delta-sigma modulator 102 receives an input signal of +4.9, a probability that the DS modulator will attempt to provide a series of output signals q representative of an output  greater than +5 exists. Such an output can cause nonlinear feedback to the loop filter. For a P-order delta-sigma modulator, the greater P is the less tolerance delta-sigma modulator 102 has for input signals close to the reference levels.
Instability of delta-sigma modulators causes many undesirable effects. In audio systems, instability can cause oscillations resulting in undesirable, audible tones. Instability can also cause abrupt signal magnitude and frequency changes, which also result in undesirable noise.
Many techniques exist for handling quantizer overload conditions. A limiter can be used to confine the input signal to a no-overload region through clipping operations in analog systems or through bit truncations in discrete systems. Other techniques rely on complicated designs, which can require a significant amount of design time, chip real estate, and implementation difficulties. However, despite an enormous amount of effort, due to the analytical complexities of high order delta-sigma modulators, accurate instability prediction mechanisms have yet to be developed.
In one embodiment of the present invention, a delta-sigma modulation system includes an M-order filter to process input data, wherein M is greater than or equal to 3. The delta-sigma modulation system further includes an N-order filter, wherein N has a value that provides stability to the delta-sigma modulator during overload conditions. The delta-sigma modulation system further includes a quantizer system coupled to the M-order and N-order filters to receive input data from the M-order and N-order filters, provide quantized feedback data, qM, to the M-order filter, provide quantized feedback data, qN, to the N-order filter, and provide 1-bit quantization output data q, wherein q=qM+qN.
In another embodiment of the present invention, digital signal processing system having a delta-sigma modulator with stability protection during quantizer overload conditions. The system includes an M-order loop filter to process a sum of input data and feedback data, qM, wherein M is more than two, and includes an N-order loop filter to process feedback data, qN, wherein N is selected from the group consisting of one and two. The system further includes a rules based 1-bit quantizer to process output data from the N-order loop filter and M-order filter and to provide qM, qN, and 1-bit quantized output data, q, wherein q=qM+qN, and qMmax is greater than the maximum value of q and qMmin is less than the minimum value of q to maintain stability of the M-order loop filter during overload conditions.
In another embodiment of the present invention, a digital signal processing system includes an M-order filter to process input data, wherein M is greater than 2 and an N-order filter that is stable during overload conditions. The system further includes a quantizer system coupled to the M-order and N-order filters to receive input data from the M-order and N-order filters, provide quantized feedback data to the M-order filter, provide quantized feedback data to the N-order filter, and provide two state quantization output data, wherein the quantization output data approximately equals the feedback data to the M-order filter plus the feedback data to the N-order filter.
In another embodiment of the present invention, a method of maintaining stability of a 1-bit delta-sigma modulation system under overload conditions includes quantizing output data of an M-order filter, wherein M is greater than or equal to 3. The method further includes quantizing output data of an N-order filter, providing feedback data, qM, to the N-order filter, providing feedback data qN to the N-order filter, and providing 1-bit quantization output data, q, wherein q equals qN+qM.
In another embodiment of the present invention, a method of maintaining stability of a 1-bit delta-sigma modulator during overload conditions, wherein the 1-bit delta-sigma modulator comprises a quantizer, an M-order loop filter, and an N-order loop filter includes providing 1-bit output data, q, based on input data from an M-order loop filter and an N-order loop filter, wherein M is greater than or equal to three and N is selected to provide stability to the delta-sigma modulator during overload conditions. The method further includes detecting an overload condition of a quantizer and providing appropriate feedback data, qM, for the M-order loop filter to enable the M-order loop filter to remain stable during the overload conditions, wherein a maximum value of feedback data qM is greater than the maximum value of output data q. The method also includes providing compensating feedback data, qN, for the N-order loop filter to maintain an acceptable gain level of the quantizer.